Text layout performance

LibreOffice Conference 2018 (https://conference.libreoffice.org/2018/the-program/sept-18th-friday/), Tirana

Transcript

1. Collabora Productivity
   www.collaboraoffice.com
   Collabora Productivity
   Text layout
   performance
   By Miklos Vajna
   Software Engineer at Collabora Productivity
   2018-09-28
   

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2. Collabora Productivity
   LibreOffice Conference 2019, Tirana | Miklos Vajna 2 / 5
   Motivation
   Quoting the TDF wiki:
   ●
   All over the code base assumes shaping text is cheap
   ●
   We can do it over and over again
   ●
   Want to measure the text? Shape it and discard the output
   afterwards. Want to measure part of the same text? Shape
   again. Want to find line breaks? Shape again. Want to finally
   draw it? Shape again.
   ●
   In contrast, other toolkits typically calculate the layout
   explicitly:
   ●
   e.g. GtkLabel has an explicit PangoLayout, etc.
   

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3. Collabora Productivity
   LibreOffice Conference 2019, Tirana | Miklos Vajna 3 / 5
   Work done: shared code
   vcl
   ●
   StatusBar (thanks to Tamás Bunth)
   ●
   ListBox
   ●
   Menu
   svtools
   ●
   Ruler
   Not a cache: just lazy-calculate the layout
   

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4. Collabora Productivity
   LibreOffice Conference 2019, Tirana | Miklos Vajna 4 / 5
   Work done: Writer
   Good news
   ●
   Everything goes through SwFntObj
   ●
   Can cache GetTextArray(), DrawTextArray(), GetTextWidth(),
   GetTextBreak() calls on OutputDevice
   Bad news
   ●
   Need a cache: SwFntObj typically doesn’t have a good idea about the
   lifecycle of the created text layout
   ●
   For now: SwTextGlyphsKey stores which subset of which text was laid
   out on which output device
   ●
   Storing the full output device state is too expensive
   ●
   Can’t handle everything transparently in VCL
   

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5. Collabora Productivity
   LibreOffice Conference 2019, Tirana | Miklos Vajna 5 / 5
   Results
   Metric: # of layout calls
   ●
   GenericSalLayout::LayoutText()
   Measured: one Writer scenario (equivalent of UITests in sw)
   ●
   55793 layout calls
   initially
   ●
   18126 layout calls
   now
   ●
   Only 32% of the
   original work is
   performed
   baseline sw draw cache ruler cache sw break cache listbox cache
   0
   10000
   20000
   30000
   40000
   50000
   60000
   # of layouts
   

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